1. Field of the Invention
The present invention is directed to a nondestructive inspection system for multilayer articles. More particularly, the present invention is directed to an ultrasonic inspection device for laminated plastic articles.
2. Description of the Related Art
Many products in use today are composed of a plurality of individual layers. This is especially true for plastics, which are often coextruded in plastic sheets having a plurality of plastic layers. In such sheets, the layers may be formed from different materials and have different thicknesses. An appropriate combination of layers yields a product having the desired properties. For example, plastic sheets which are used to form food containers are typically multilayer in nature. These sheets can have many layers, with five or more not being uncommon. In plastic sheets used for food containers, one of the interior layers preferably functions as an oxygen barrier layer. The presence and quality of this layer have a direct bearing on the shelf life of the food product. This layer is also the most expensive to produce. One layer, for example the top layer of a sheet, has characteristics which enable it to withstand handling, while another layer, for example the bottom layer of the sheet, is designed for contacting the food product without degrading it. Intermediate layers can include tie layers for tying the top and bottom layers to the oxygen barrier layer. Regrind layers, which are made from scrap material and are therefore less expensive to produce, may also be included to function as filler layers. Such sheets can have a total thickness of, for example, from 5 to 120 mils, with the thickness of individual layers, such as the oxygen barrier layer, being as little as one mil or less.
Given the thinness and importance of such layers, it is necessary in the manufacturing process to diligently monitor layer parameters. Accordingly, quality control and sheet inspection are very serious concerns. However, no system proposed to date has proven sufficiently reliable to replace inspection by hand. Inspection by hand is labor intensive, slow, and quite costly. Typically, a sample is mechanically shaved from an edge of a coupon cut from a sheet to provide a cross-section of the sheet. The sample layer thicknesses are then optically measured under a microscope.
As can be appreciated, this inspection technique has a number of drawbacks. First of all, each article to be tested is physically invaded. While this might not matter for some laminated products, such damage to other laminated articles can render the product partially or entirely unusable. Further, in order to provide appropriate quality control, large numbers of samples must be taken. Since each sample typically takes approximately fifteen minutes to inspect, it can take one or more hours to inspect a sheet. This results in valuable production time being lost if production is held up until the quality results are verified, and the physical invasion of the article is increased. Alternatively, if production is continued while the sample is being inspected, the manufacturer runs the risk that a defective product is being produced if the test results indicate that the sample is unacceptable. Additionally, the quality of the inspection by hand itself is suspect, since the process is a multistep, labor intensive process which depends upon human accuracy in each step. Similarly, due to the time it takes to obtain results, the number of samples that can be taken is limited, thereby endangering quality control, as quality problems away from these limited areas or sampling points will not be identified using this technique.
Nondestructive inspection techniques are now used to inspect a wide variety of articles. Many of these techniques involve the transmission of ultrasonic energy into an article to be inspected. Typically, ultrasonic energy is generated by a pulser/receiver which produces an electrical pulse that excites a piezoelectric or magnetostrictive transducer, causing the transducer to emit an ultrasonic pulse. In a "pulse-echo" technique, this ultrasonic pulse travels into the article under inspection until it is reflected from an interface or internal flaw. The reflected pulse is received and converted by the transmitting transducer into an electrical signal for analysis. Another technique involves the "through transmission" of ultrasonic energy through an article to be inspected. In this technique, a signal is transmitted by a transmitting transducer, goes through the article under inspection and is received by a receiving transducer. The pulse received by the receiving transducer is converted into an electrical signal for analysis.
Such systems are able to detect, locate and record defects in the product, or the presence or absence of certain components. For example, U.S. Pat. No. 3,575,043 to Allen et al. relates to an ultrasonic inspection system for detecting defects in multicomponent assemblies. The system is capable of detecting bonding voids in the nature of approximately 1/16 inches in diameter and larger. A pair of transducers are mounted on opposite sides of a nonuniform article to be tested. As the ultrasonic transducers are moved along the surface of the non-uniform article, such as a helicopter blade, ultrasonic pulses are transmitted and received, and a C scan image is formed on recording paper. Flaws are indicated by lack of recording on the recording paper by a stylus.
U.S. Pat. No. 3,505,859 to Byers employs transmitting and receiving transducers to send ultrasonic energy through a metal object immersed in a liquid. The metal object is moved relative to the transducers during testing. If a defect is found, the article is rejected.
U.S. Pat. No. 4,587,849, which issued to Gross, employs a single transducer mounted on a production line for detecting the presence and depth of interfaces in a coextruded plastic sheet. A chamber mounted between the face of the transducer and the sheet holds an interfacing fluid through which ultrasonic pulses pass to and from the sheet.
However, nondestructive inspection systems have failed to replace the hand testing technique in measuring layer thicknesses in multilayer articles and/or in articles including extremely thin layers. It is believed that nondestructive inspection systems have proven ineffective when attempts have been made to employ them for these purposes. Accordingly, a need has arisen for a nondestructive inspection system which is capable of detecting the thicknesses of a product having a plurality of layers and/or very thin layers.